The present invention relates to methods and apparatus for sanitizing items. More particularly, the invention relates to improved microwave cooking systems having a plurality of linearly aligned segments for processing food products.
The invention finds special, but not exclusive application in the sector of collective catering, where sterilization treatment of foods already sealed in containers not to be consumed immediately is required. A second possible application can also concern sterilization or sanitization of other products intended for the food chain, like flour, rice, as well as specific products of various nature, prepared or not, and medicinal products or parts of them. Still a third application of the present invention concerns the sterilization of medical equipment.
Techniques for conditioning foods for serving of meals to a large number of persons, for example, are certainly known, as occurs in dining halls, in hospitals and other facilities, where large numbers of persons make traditional catering untenable, at least in terms of cost. On the practical side, these techniques can be summarized in three basic steps: a) selection and precooking of foods; b) preservation; and c) serving.
Conventionally, a cycle of selection and precooking of foods is followed by a preservation cycle, which typically includes the use of refrigerators or freezers and, in more recent techniques, rapid heating vessels.
In some cases, where preservation on an industrial scale is required, a post-preparation sterilization phase is required between the first and second stages, which, as in the case of use of a container alone, is not limited to attenuation of microbial, pathogenic and enzymatic activity, but has the purpose of destroying all microorganisms present in the product, and also in the actual container/package. This occurs, because the degree of resistance to heat of microorganisms is related to external and environmental factors, like the initial microbial concentration of the medium, the characteristics of the medium itself and the time and temperature parameters, as well as intrinsic factors related to heat sensitivity of germs, development stage of the cells, in which specific variations often occur. For example, under identical environmental conditions, it is observed that fungi and yeast are more resistant than coli bacteria and, within the latter, the rod forms are more resistant that the coccal forms.
Under practical conditions, to carry out sterilization, it is necessary to heat the product to a temperature between 65° C. and 121° C. for a time of between 5 and 12 minutes. Subsequently, the product must be subjected to the most rapid possible cooling to a temperature equal to or less than 35°.
The use of high frequency electromagnetic waves, better known as microwaves, is known for performing the sterilization stage. In this sense, GB1103597 (Newton et al.) already suggested a system for controlling microorganisms contains in prepared foods and beverages. It prescribes for exposure of the already prepared foods with the package to electromagnetic waves with a frequency of 20-40 MHz at an intensity of 500-3000 volts for a sufficient period of time to attenuate the microorganisms present in the manufactured product. The use of a magnetron to sterilize materials is known in even greater detail. For example, WO0102023 (Korchagin) proposes a magnetron that has the capacity to implement the intensity of the magnetic field at a level to ensure destruction of microorganisms.
Complex apparatuses, specifically continuous treatment tunnels for sanitization of packaged products, have been known since 1973. U.S. Pat. No. 3,747,296 (Zausner) proposes an apparatus with linear development, in which filled containers are introduced and subsequently closed. Said containers are passed through the tunnel, which is subdivided into different treatment zones at temperatures between 90° C. and 150° C. Means of irradiation are also provided, which have the purpose of sterilizing the cover only.
U.S. Pat. Nos. 5,066,503; 5,074,200; 5,919,506 and 6,039,991 issued to Ruozi describe conveyor driven microwave processing plants for pasteurizing, cooking and sterilizing food products. The plants include a plurality of chambers wherein the temperature and pressure are controllable elevated and decreased within as the food products travel from chamber to chamber.
U.S. Pat. No. 3,889,009 (Lipoma) describes a conditioning tunnel for foods previously prepared in bowls and sealed under pressure. The conditioning tunnel essentially consists of an external covering, along which a conveyor belt moves. At the entry and exit of this tunnel, corresponding to the crossing point of the manufactured vessels, pressure closure doors are provided. Once the sealed vessels have entered the interior of the tunnel, each vessel undergoes a sterilization treatment, passing beneath a source of electromagnetic waves. Each vessel is then transferred downline, always by means of a common belt or chain conveyor, to pass through a cooling unit. A device to generate pressure during the sterilization phase operates within the apparatus to avoid a situation in which the products, because of the process, burst because of the dilation effect, or whose sealing strength is altered. This phenomenon most frequently entails escape of liquid from individual containers, producing not insignificant drawbacks within the apparatus, like accumulation of dirt and the subsequent need to carry out frequent maintenance.
Other apparatuses based on developments of the system just described are also known. For example, in the catalogs of the Italian companies Modo Group International from Brescia Italy and Micromac from Reggio Emilia, automatic and computerized tunnels are described, which provide for receiving the products, in this case prepared dishes in a heat-sealed vessel, and are designed to carry out the fundamental phases of sterilization treatment. The tunnels include elongate cylindrical constructions have diametrically round cross sections, within which, corresponding to the different stages, the following process phases are conducted: 1) preheating; 2) reaching the sterilization temperature by means of induction devices that generate microwaves; 3) holding or stabilization of the product at the sterilization temperature for a specified time (magnetrons, which are positioned along the lower side of the conditioning tunnel beneath or corresponding to the plane of advance of the prepared foods, are typically provided to execute at least these last two phases); and 4) cooling before unloading. At the end of the process, a finished product emerges, completely sanitized and ready to be packaged and stored in warehouses.
Unfortunately, the prior art food processing systems suffer from numerous disadvantages. In particular, the previous solutions provide for the necessary magnetrons for gradual reaching and maintenance of the temperature within each product. These devices are situated indifferently along the overlying or underlying side of the line of advance of the heat-sealed bowls/trays/vessels. The cross section of the known conditioning tunnel is round, so that this circumstance actually limits the number of magnetrons that can be located along the axes perpendicular to the direction of advance of the vessels. Consequently, this shortcoming gives rise to two significant defects, in the first place excessive dimensioning (especially in length) of the treatment apparatus, and, when one intends to keep the dimensioning equal to the treated amounts, requiring additional microwave cooling devices, which are particularly expensive, and also difficult to operate and maintain.
Invariably in the known solutions, owing to the fact that the cross section of the conditioning tunnel is round, the means of longitudinal transport with respect to the tunnel, in the present case a belt or chain, is always contained within the tunnel. This second circumstance actually limits the space available within the tunnel, ultimately reducing the necessary area for treatment of the material. In addition, the presence of a conveyor device almost completely within the tunnel, with all the electromechanical mechanisms necessary for its functioning, offers an infinite number of surfaces and receptacles that are difficult to reach, within which dirt tends to progressively accumulate. The problem is a recurrent one, because the products being treated are, for the most part, food products with frequent presence of liquid, which can also be accidentally dispersed within the tunnel. These events require the use of frequent maintenance, in order to keep the qualitative aspect of treatment high.
Further problems are associated with the characteristics of the non-return valves that divide each of the stages present along the tunnels of the traditional type. These valves are of the mechanical opening and closing type, whereas the movement that they execute is essentially along a linear axis, using fittings situated peripherally to the closure plate. The negative aspect of these solutions concerns the fact that they are fairly complex and require accurate and constant maintenance to ensure, between the different treatment stages, maintenance of the pressure present in the concerned section.
Finally, it can happen that during sterilization treatments, in this case, heat-sealed vessels, some of them can burst, dispersing the liquid into the surrounding area. At present, on occurrence of the event, it is necessary to ensure the correct treatment, stop the installation and carry out thorough cleaning with removal of the leaked material.